"I was sitting in the house and I heard a big bang," Mike McGraw told KYW. "The door slammed and the TV went out."

His son, Mike McGraw Jr., said he saw the buckled buildings and watched firefighters hose down a man who had been burned.

"They threw this baby out the window because the girl was on fire," he said, "and the fireman had to catch the baby."

A contractor was doing rehab work in a vacant home when it collapsed, Deputy Fire Chief Robert Coyne told reporters. The two adjacent houses were also destroyed, he said.

One person was in critical condition with severe burns, city officials said.

A spokesperson for Jefferson University Hospital told the CNN affiliate that seven people were being treated for minor injuries there, including a mother, a father, and a two-month-old baby who were inside the home next to the house that collapsed.

Dozens of homes remained evacuated Monday afternoon. City officials said all residents in the area had been accounted for, but fire officials were using listening devices to double check.

Aerial footage of the scene showed a large pile of rubble between two buildings, with splintered wood beams sticking out where rooms of the house once stood.

Joe Szymborski told KYW that he was lying in bed when he heard the explosion.

"I threw on my shoes, ran down there and helped three people out of the house," he said.

"It's just instinct. Anybody would do it for anybody else around here," he said. "We're all neighbors and friends."

The home where the blast occurred was a permitted construction site, city officials said. Authorities are investigating to determine what caused a gas leak in the area. Gas service on the street has been shut off, the city said.

Last month, a four-story wall of a vacant building in Philadelphia collapsed onto an adjacent Salvation Army thrift store, killing six people and injuring 13.